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Full text of "The Cambridge Natural History"

iv                                         PLAGIAUJLAX                                       99
Theromorphous group). (2) The Triconodonta, which were
Marsupials, though in all probability with a complete succession
of teeth and with an allaritoic placentation. This group will
include the genera Phascolotherium and Amphilestes, as well as
Triconodon and SpalacotJieriufn. Finally we have (3) the Tritu-
berculata (or Insectivora Primitiva) with the genera A-inpliiiheriuyn,
Perarnus, AmblotTierium, Stylacodon, and Dryolestes.
We shall take these three groups in order. The Multituberculata
have already been to some extent denned, if such a word can be
used to express the summation of the very scanty information at
our disposal. Of this group, Plagiaulax is a genus which occurs in
the Purbeck beds; it is only known by lower jaws implying an
animal of the size of a Eat or rather smaller. The jaws have in
front a large incisor which looks Kodent-like, and also like those of
the Diprotodont Marsupials ; but it is held that these teeth did not
grow from persistent pulps, and there is in any case no anterior
thickened coating of enamel. Canines are absent; the diastema
is followed by four premolars increasing progressively in size and
possessing somewhat complicated grinding surfaces. These surfaces
are formed by several obliquely-set ridges. The succeeding teeth
are termed molars on account of their difference in structure, and
bhere are but two of them on each side. The molars are of a
pattern common in the Multituberculata; the centre is hollowed,
ind the raised rim is beset by tubercles. Other Jurassic genera
>f Multituberculates are Bolodon, Allodon, and titereognathus.
^Lll of these possess the same rnultituberculate molars.
Of the Triconodonta the type-genus is Triconodon.     This genus
s better known than most Jurassic mammals, since both the upper
tnd  the lower dentition have been described.     It appears to have
>ossessed the typical Eutherian dentition of forty-four teeth, to
rhich a fourth molar is added in some species.     The great differ-
nce between the molars and premolars argues a complete tooth-
hange.     The genus is American as well as European.
Spala^otJieriu'm has more molars, five or six.
Phascolotherium   'bucklandi,   on  the   other hand,   is   a  much
Ider type in the form of its teeth.     There are, however, not so
lany of them as in AmpMtherium ;  PJiascolptherium has but two
remolars and five molars, making a total of forty-eight teeth.
lhe teeth are of the triconodont form, the three cusps being in
ne, and the middle one the largest.